This invention relates to a mounting system for a heat exchanger for use on a gas turbine engine.
The use of gas turbine engines in stationary applications and in ground moving vehicles permits the inclusion of a heat exchanger which serves the purpose of heating incoming air by extracting heat from the exhaust gasses. Gas turbine engines operate most efficiently at the higher temperatures, thus preheating the intake air prior to injecting fuel results in a higher output efficiency per pound of fuel burned. Therefore, with the inherent characteristic of high exhaust temperatures in gas turbines, it has become common when weight is not a critical factor to add a heat exchanger to extract excessive heat from exhaust gasses to heat inlet gasses. To increase efficiency even further, it is the normal procedure to heat the inlet gasses subsequent to compression. Thus the inlet gasses are ducted from the compressor section of the engine casing through a heat exchanger and then returned to the engine in the vicinity of the combustion system through a second duct. A third duct interconnects the exhaust from the engine to the heat exchanger. The heat exchanger provides two separate flow paths with the flow paths being separated by thin walls. An example of the heat exchanger contemplated for use in this type engine is described in U.S. Pat. No. 3,759,323.
The material used for construction of the heat exchanger and the material used for construction of the engine generally will have different thermal expansion characteristics, not to mention the fact that there are varying thermal expansion characteristics in the heat exchanger itself. Accordingly, one of the problems in designing a heat exchanger and an associated mounting system for a gas turbine engine has been to overcome the problems inherent in different thermal expansion rates for the two components. In order to achieve the most efficient type heat exchanger, it has been found appropriate to mount the heat exchanger immediately adjacent the engine, generally above the engine. A typical mounting for a heat exchanger adjacent the engine is disclosed in U.S. patent application, Ser. No. 548,112 now U.S. Pat. No. 3,968,834 assigned to the assignee of this invention. The mounting means disclosed therein has proved satisfactory for use in lower horsepower engines but as the horsepower increases the thermal expansion problems increase, particularly in relation to the innerconnecting manifolds between the heat exchanger and the engine. Of the various structures disclosed in the above-mentioned mounting means application, movement between the heat exchanger and the engine generally occurs at all three manifolds; this movement can be both lateral and vertical relative the engine. Fixing the center of the three manifolds when the manifolds are serially placed, results in movement of the other two manifolds in opposite directions as the temperature increases or decreases. Accordingly, a flexible joint must be provided at at least two manifolds and further a second support means provided for the heat exchanger if the fixed manifold is to act as a support means. The second support means must itself be flexible in order to allow motion of the heat exchanger relative the engine. In higher horsepower engines it has been found that fixing a manifold is unsatisfactory as there will always be a certain degree of motion of the manifold itself due to thermal expansion, but to provide two support means neither associated with a manifold has at least two drawbacks. First of all it increases the weight and cost of manufacture of the engine in the design and construction of the second support means and secondly it requires that all three manifolds provide for both vertical and longitudinal movement relative the engine.
A problem which has been encountered in the past in heat exchanger systems utilizing a flexible gas-in duct such as that described in U.S. patent application, Ser. No. 548,112 is reactive pressure loading in the gas-in duct and to a limited extent in the gas-out duct. The increase in pressure caused by the compressed air from the engine being delivered to the heat exchanger results in the bellows type connection in addition to expanding due to heating having a further expansion due to increased pressure in the heat exchanger system itself. The net result of the heat expansion and the reactive pressure loading is to lift the heat exchanger upwardly of the engine case particularly when a three bellows system is utilized. When one of the manifolds acts as a support point, rotation occurs at that support point with expansion due to heat and pressure loading forcing at least one of the other manifolds upwardly away from the engine.